1. Field of the Invention
The present invention relates to evaluating stress on a surface without contacting the surface.
2. Description of the Related Art
Many advanced defense missile systems use an infrared (IR) seeker for the purpose of identifying and tracking the intended target of interest. Due to the nature of the aerothermal flight environment, the protective IR transparent window must be able to survive extremely high thermal stresses ( greater than 100 MPs) in order to prevent catastrophic failure. In many missile systems, the window material of choice has been crystalline sapphire, which has both optical and mechanical properties that are suitable over a wide range of operational flight conditions.
To assure the safe operation of a seeker window, the performance of the IR window under realistic stress and temperature conditions typically is examined. In this examination, the threshold limits of the material making up the window may be determined. Since the conditions encountered in use typically are extreme, the same conditions typically are encountered in testing. Testing often occurs in a wind tunnel. However, the measurement of sapphire window strain in hypersonic wind-tunnel applications is very difficult. Aerothermal heating and shear usually preclude the mounting of common strain gauges on the front side of windows under test. Back-side mounting is complicated by the extreme temperatures commonly seen by these windows.
In many test simulations, sample temperatures can easily exceed 500 degrees C. and can extend to 1000 degrees C. Temperatures of this magnitude prohibit the use of conventional, direct-contact strain gauge transducers. Along these lines, strain gauge adhesives typically break down at temperatures in the vicinity of 320 degrees C.
Mounting strain gauges on the back-side of the windows also does not allow the measurement of front-surface stresses. The physical size of strain gauges reduces spatial resolution and does not allow for a high density of measurements. Strain gauges are intrusive and can affect thermal gradients and, thereby, local strain gradients on the material under test. Crystalline windows are also commonly used with corrosives where strain gauges are attacked by the surrounding media.
Optical fluorescence provides an alternative approach to direct contact probing. Optical fluorescence relies on the ability to generate emission from ions such as chromium, magnesium, and vanadium that are embedded in a crystalline lattice of window materials. For example, it is known that chromium ions in crystalline sapphire produce a narrow-band fluorescence doublet in the red region of the spectrum. The doublet is sensitive to both temperature and stress in the sample. These two intense emission lines are termed the R-fluorescence lines.
The effect of an applied stress to a sapphire window is the distortion of the crystal field surrounding the chromium ion. The distortion changes the potential energy of the ion and, hence, the emission wavelength of the fluorescence radiation. Thus, the effect of stress can be quantitatively calibrated as a shift in the characteristics of the R-fluorescence lines and used as a non-contact probe of stress in sapphire windows.
The present invention provides a non-contact method for evaluating stress in a substrate. The method includes non-uniformly introducing at least one impurity into the crystalline substrate. The crystalline substrate is subjected to physical stress. Fluorescence producing energy is directed at the crystalline substrate. A fluorescence produced by the crystalline substrate is measured. The fluorescence spectrum is correlated with the stress on the crystalline substrate.
The present invention also includes a method for manufacturing a structure for non-contact evaluation of stress in the structure. According to the method at least one impurity is non-uniformly introduced into a crystalline substrate.
Additionally, the present invention provides a structure for non-contact evaluation of stress in the structure. The structure includes a crystalline substrate including at least one impurity non-uniformly distributed in the substrate.
Furthermore, the present invention provides a device for non-contact evaluation of stress in a substrate. The device includes a hollow cylindrical window support operable to support the substrate. A source of fluorescence producing energy is operable to direct the fluorescence producing energy at the substrate. A heat source is operable to subject the substrate to elevated temperature. A mechanical loading assembly is operable to subject the substrate to a mechanical load. A sensor is operable to detect fluorescence emitted from the substrate.
Still further, the present invention provides a non-contact method for evaluating stress in a sapphire window. The method includes subjecting to a physical stress a sapphire window that includes at least one impurity non-uniformly distributed in at least one region in the vicinity of at least one surface of the sapphire window. Fluorescence producing energy is directed at the sapphire window. A fluorescence produced by the sapphire window is measured. The fluorescence spectrum is correlated with the stress on the sapphire window.
Still other objects and advantages of the present invention will become readily apparent by those skilled in the art from a review of the following detailed description. The detailed description shows and describes preferred embodiments of the present invention, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the present invention is capable of other and different embodiments and its several details are capable of modifications in various obvious respects, without departing from the present invention. Accordingly, the drawings and description are illustrative in nature and not restrictive.